A thermal cycling experiment was performed on GaAs solar cells to establish the electrical and structural integrity of these cells under the temperature conditions of a simulated low-Earth orbit of 3-year duration. Thirty single junction GaAs cells were obtained and tests were performed to establish the beginning-of-life characteristics of these cells. The tests consisted of cell I-V power output curves, from which were obtained short-circuit current, open circuit voltage, fill factor, and cell efficiency, and optical micrographs, spectral response, and ion microprobe mass analysis (IMMA) depth profiles on both the front surfaces and the front metallic contacts of the cells. Following 5,000 thermal cycles, the performance of the cells was reexamined in addition to any factors which might contribute to performance degradation. It is established that, after 5,000 thermal cycles, the cells retain their power output with no loss of structural integrity or change in physical appearance

Francis, R. W.

Janousek, B. K.

Wendt, J. P.

English

NASA Technical Reports Server

N'86 - 17865
THERMAL STRESS CYCLING OF GaLAs SOLAR CELLS
Bruce K. Janousek, Robert W. Francis, and Jerry P. Wendt I
The Aerospace Corporation
Los Angeles, California
I. Introduction
A thermal cycling experiment is being performed on GaAs solar cells to
establish the electrical and structural integrity of these cells under the
temperature conditions of a simulated low-earth orbit of 3-year duration
(15,000 cycles from -80°C to +80°C). Thirty single junction GaAs cells were
obtained (ten each from Applied Solar Energy Corporation, Hughes Research
Laboratories, and Varlan Associates) and tests were performed to establish the
beginnlng-of-life characteristics of these cells. These tests consisted of
cell I-V power output curves, from which were obtained short-circult current,
open circuit voltage, fill factor, and cell efficiency, as well as optical
micrographs, spectral response, and Ion microprobe mass analysis (IMMA) depth
profiles on both the front surfaces and the front metallic contacts of the
cells. Following 5,000 thermal cycles, the performance of the cells was re-
examined in addition to any factors which might contribute to performance
degradation. The results presented here establish that, after 5,000 thermal
cycles, the cells have retained their power output wlth no loss of structural
integrity or change in physical appearance.
II. Be_Innlng of Llfe Cell Characteristics
The thirty GaLAs solar cells obtained for this experiment were 2 x 2 cm,
p-on-n, unglassed, slngle-junctlon cells wlth thicknesses of either 12 or 15
mils. To fully characterize these cells the following tests were carried
out: I) I-V measurements, 2) spectral response, 3) IMMA depth profiles, and
4) optical microscopy.
A. Cell I-V Curves
The cell I-V curves were measured at the Jet Propulsion Laboratory
(JPL) using a Spectrolab X-25 solar simulator in conjunction with a balloon-
flown GaAs standard cell to establish beginning-of-life efficiency and to
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£he_ e _clenctes reported by the vendors. All cells loaded into the
"temperature _ cycler had a beginning-of-life AM0 efficiency greater than 16%
aside from one cell with a measured efficiency of 15.95%. The average
efficiency of the combined thirty cells was 16.66% with a standard deviation
of 0.53%.
B. Cell Spectral Response
Absolute spectral response measurements were obtained at JPL on two
solar cells from each of the three vendors. These curves allow one to follow
the factors which might contribute to loss in performance by determining which
region of the cell is degrading. The beginnlng-of-llfe cell spectral response
curves all show the classical behavior exhibited by GaAs solar cells - a sharp
rise near the bandgap wavelength (~ 900 nm) to a maximum response of _ 0.55
mA/mW followed by a gradual decrease in response at shorter wavelengths and a
sharp drop at wavelengths less than 450 nm.
C. Ion Microprobe Mass Analysis
One of the possible degradation mechanisms in GaAs solar cells is
the diffusion of the front contact metallization into the junction region of
the cell with subsequent cell shorting. Thus, elemental depth profiles were
obtained employing an ARL ion microprobe mass analyzer (IMMA) on the front of
one each of the vendors' cells both on and between the grid lines. By
obtaining IMMA depth profiles before and after temperature cycling, one can
determine whether metal diffusion is occurring.
IMMA sputtering was carried out on the tip of a grid llne furthest
from the cell bus bar to minimize the possibility of degrading the cell
output. The investigated cells demonstrated both well-behaved graded
metallization profiles and semiconductor interface regions.
D. Optical Micrographs
Color photographs at a magnification of 4X _ere taken of all 30
cells in order to compare the cell surface morphology, optical properties, and
possible grid line delaminatlon before and after thermal cycling.
III. Temperature Cycllng
Temperature cycling is being performed in The Aerospace Corporation's
232,
Aerophysics Laboratory. The cycle period is 30 minutes, and the temperature
extremes are -80°C and +80°C with a sinusoidal temperature vs. time
relationship. During cycling, the cells are maintained at a pressure less
than 10-6 Torr. The cycler includes fail safe features which prohibit the
solar cell from experiencing temperatures above +I00°C and below -IO0°C. In
the event of a loss of vacuum, the cells are returned to room temperature.
During cycling, the solar cells sit in an aluminum "picture frame" which
is bolted to the temperature-controlled cooling/heating block. The picture
frame has 36-1.0 inch square openings to accommodate the cells. Three 2 x 2
cm silicon solar cells are included in the thermal cycling test to provide an
internal standard for comparison to the GaAs thermal stress results. The
remaining three openings are filled with electrically-inactive GaAs cells with
thermocouples attached with conductive epoxy to allow temperature monitoring
inside the cell block. In addition, three thermocouples are epoxied to the
outside of the temperature block to provide control and monitoring of the
temperature and thermal gradients. A cover plate over the solar cells
encloses the cells while they are in thermal contact with the cooling/heating
block such that no light reaches the cells during the temperature cycling.
Table I describes the monitored thermal data for the initial 497 thermal
cycles; this data is indicative of the thermal environment for the subsequent
4,503 cycles. T6 and T8 are thermocouples affixed to the outside of the
cooling/heatlng block on each end and T7 is attached to the outside middle of
the block. The thermocouples attached to the GaAs blanks inside the block
were not employed for temperature control or monitoring since the
thermocouples became disconnected from the GaAs surface during cycling,
resulting in anomalous temperature readings. Prior to this, however, it was
established that the temperatures measured inside the block on the GaAs blanks
were representative of the temperatures measured on the outside surface of the
block. The maximum low temperature (-I12.4°C) was achieved at T8 when a
solenoid valve temporarily stuck open, allowing liquid nitrogen to enter the
cooling/heating block from both sides.
IV. Temperature Cycling Results
A. Cell I-V Curves
Data comparing cell performance before and after the 5,000 thermal
233
cycles are presented in Table 2. The solar cells are listed in descending
order of beglnnlng-of-llfe efficiency. Two cells, #9 and #19, were damaged
during IMMA analysis, resulting in a performance degradation. Thus, the
averages listed at the bottom of Table 2 reflect those taken on the beginning-
of-life performance and the 5,000 thermal cycle performance excluding cells #9
and #19.
The data in Table 2 indicate that the performance change of the GaAs
solar cells after 5,000 thermal stress cycles is very small and within the
experimental error of the measurement (± 0.20%). The small decrease in
efficiency observed (16.67% to 16.60%) was largely due to a decrease in
average cell fill factor from 0.781 to 0.777. The small decrease in average
Voc was almost identical, on a percentage basis, to the small increase in
average Isc. The average efficiency of the three Si cells included for
comparison in the temperature cycling test increased from 13.44% to 13.50%
after 5,000 cycles, an increase which is again within the experimental error
of the efficiency measurement.
B. Cell Spectral Response
Absolute spectral response curves after 5,000 thermal cycles for the
six cells on which spectral response measurements were obtained before cycling
showed negligible changes due to the thermal stress.
C. Ion Microprobe Mass Analysis
Ion Microprobe Mass Analysis depth profiles obtained after the 5,000
thermal cycles between and on the metal grid lines on one each of the vendors'
cells showed no interface redistribution or enhanced penetration of slntered
metal contacts due to the thermal stress.
D. Optical Micrographs
Optical mlcrographs taken after 5,000 thermal cycles showed no
change in the surface morphology or optical properties of the solar cells.
The grid llne of one cell peeled along a short section (~ 0.7 mm) furthest
from the bus bar; this may have caused the fill factor decrease for this cell
from 0.814 to 0.799.
234
V. Summary/Conclusions
Temperature cycling of single junction GaAs solar cells to simulate the
temperature conditions of a low-earth orbit of 3-year duration has been
initiated. The change in cell electrical performance after 5,000 thermal
stress cycles was found to be negligible. Furthermore, there were no observed
changes in the cell spectral response, metallization and interface profiles,
surface morphology, and optical characteristics. These results should enhance
the overall confidence in GaAs solar cells for space applications since the
cells tested in this experiment represent different crystal growth and
metallization schemes. The cells are currently undergoing the second set of
5,000 thermal cycles, and performance will again be evaluated after 10,000 and
15,000 thermal cycles.
Vl. References
1. The authors gratefully acknowledge the cooperation of Applied Solar
Energy Corporation, Hughes Research Laboratories, and Varian Associates for
supplying solar cells for this experiment. We would also like to thank Bruce
Anspaugh and Bob Weiss of the Jet Propulsion Laboratory for their assistance
in carrying out the cell efficiency and spectral response measurements, Martin
Lundquist and Tim Wall for the design and operation of the temperature cycler,
and Nick Marquez for carrying out the Ion Microprobe Mass Analysis
experiments.
235
Table I. Temperature Cycling Data for the Initial 497 Thermal Cycles
Minimum Temperatures (@C)
T6 T7 T8
Average -87.1 -87.6 -93.5
o 3.6 1.7 4.5
Minimum Minimum -69.1 -85.0 -82.1
Maximum Minimum -96.7 -95.0 -112.4
Maximum Temperatures (°C)
T6 T7 T8
Average 79.7 85.0 87.2
o 2.4 2.4 3.9
Minimum Maximum 70.7 70.9 70 •2
Maximum Maximum 90 •9 95 •3 99 •4
236
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Thermal stress cycling of GaAs solar cells

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